In a Long Term Evolution (LTE) system, Physical Downlink Control Channels (PDCCHs) are transmitted in a downlink sub-frame, and the PDCCHs have a Time Division Multiplexing (TDM) relationship with Physical Downlink Shared Channels (PDSCHs). The PDCCHs are transmitted in first N Orthogonal Frequency Division Multiplexing (OFDM) symbols of a downlink sub-frame, where N may take the values 1, 2, 3 and 4, and N=4 may be allowable to occur only in a system at a system bandwidth of 1.4 MHz, as illustrated in FIG. 1.
A control region for transmission of the PDCCHs in the LTE system is constructed from Control Channel Elements (CCEs) divided logically, where the CCEs are mapped to Resource Elements (REs) through full interleaving thereof. Downlink Control Information (DCI) is transmitted also in CCEs, and one DCI for a User Equipment (UE) can be transmitted in N consecutive CCEs, where N in the LTE system may take the values 1, 2, 4 and 8, referred to an CCE aggregation level. The UE performs PDCCH blind detection in the control region to search for a PDCCH transmitted thereto, that is, the UE attempts decoding on different DCI formats and CCE aggregation levels using a Radio Network Temporary Identifier (RNTI) thereof and receives DCI intended for it upon correct decoding. The UE in the LTE system needs to perform blind detection on a control area in each downlink sub-frame in a discontinuous reception status to search for a PDCCH.
A control region in a sub-frame in the LTE system is consisted of two spaces which are a Common Search Space (CSS) and a UE-specific Search Space (UESS), where the common search space is primarily configured to transmit DCI for scheduling cell-specific control information (e.g. system information, paging information, group-east power control information, etc.), and the UE-specific search space is primarily configured to transmit DCI for scheduling respective UE resources. A common search space in each downlink sub-frame includes first 16 CCEs, and only two CCE aggregation levels of 4 and 8 are supported in the common search space; and a UE-specific search space of each LTE in each downlink sub-frame starts with a CCE location related to the serial number of the sub-frame, the RNTI of a UE, etc., and CCE aggregation levels of 1, 2, 4 and 8 are supported in the UE-specific search space. In the UESS, blind detection at each aggregation level corresponds to a search space in which there are a plurality of PDCCH candidates, that is, blind detection by the UE at a different aggregation level is performed in a different search space. Table 1 depicts a search space in which blind detection by a UE is required in a downlink sub-frame, where L represents the index of an aggregation level, Size represents the number of CCEs in which blind detection is required in correspondence to each aggregation level, and M(L) represents the number of blind detection attempts at the corresponding aggregation level and also the number of PDCCH candidates. FIG. 2 further illustrates a schematic diagram of the blind detection procedure. As depicted in Table 1, a UE needs to attempt blind detection on 22 PDCCH candidates including 6 PDCCH candidates in a common search space and 16 PDCCH candidates in a UE-specific search space.
TABLE 1Search space Sk(L)AggregationSizeNumber of PDCCHTypelevel L[in CCEs]candidates M(L)UESS16621264828162CSS41648162
The capacity and transmission efficiency of PDSCHs in a Long Term Evolved-Advanced (LTE-A) system will be improved significantly due to the introduction of Multi-User Multi-Input Multi-Output (MU-MIMO), Coordinated Multi-Point transmission (CoMP), carrier aggregation and other schemes, and a Remote Radio Head (RRH) of an infra-cell identifier (ID), 8 antennas and other configurations; and PDCCH in the LIE-A system have not benefited from the emerging technologies for the purpose of an improvement over the earlier LTE versions (e.g., Rel-8/9).
In order to improve the performance of the LTE-A system and extend the capacity of PDCCHs, enhanced PDCCH (E-PDCCHs) have been introduced into the Release 11 (Rel-11).
In order to address a limited capacity of downlink control channels and improve the transmission efficiency of downlink control information, in one solution thereto, enhanced PDCCHs are transmitted in a PDSCH domain in a downlink sub-frame while reserving an original PDCCH domain in which existing transmission and reception schemes continue their use with original PDCCH resources, for example, transmit diversity for transmission and blind detection on DCI in a common search space and a UE-specific search space based upon a Common Reference Signal (CRS) for reception, where first N OFDM symbols are occupied for transmission, N may take the values 1, 2, 3 and 4, and N=4 may be allowable to occur only in a system at a system bandwidth of 1.4 MHz, and this part of the PDCCH domain is referred to a legacy PDCCH domain. Time and frequency resources beyond the legacy PDDCH domain are occupied for transmission in an enhanced PDCCH domain, where a part of the original PDCCH resources are frequency-division multiplexed with PDCCHs through, and this part of the PDCCH domain is referred to enhanced PDCCH domain. This solution of frequency-division multiplexing of enhanced PDCCHs with PDSCH is referred to FDM E-PDCCH, as illustrated in FIG. 3.
PDCCHs are transmitted in the prescribed modulation scheme of Quadrature Phase Shift Keying (QPSK) in the LTE Rel-8/9/10.